Implantable optical sensors have been used for monitoring relative changes in a patient's blood oxygen saturation. Acute or chronic ambulatory monitoring of a patient's blood oxygen saturation has many clinically useful applications, among which are monitoring a patient condition relating to his or her hemodynamic status, cardiac function, tissue perfusion, or respiration function. An implantable optical sensor employs at least one light source, e.g. a light emitting diode (LED), and at least one light detector, e.g. a photodiode. The light source emits light from the sensor which is reflected or transmitted through an adjacent body tissue back to the sensor. The light detector, also referred to herein as a “photodetector”, is a light sensitive device that generates a current signal proportional to the intensity of light received by the light detector. The attenuation of light emitted by the sensor, as measured by the light detector, allows a characteristic of the blood or tissue to be monitored based on changes in light attenuation by the blood or tissue over time.